1. Statement of the Technical Field
The invention concerns fiber optic devices, and more particularly, fiber optic devices utilized as a sensor for measuring a parameter of interest such as temperature.
2. Description of the Related Art
It is known in the art to utilize fiber optic devices for filtering an optical signal or measuring parameters of interest related to light propagating through an optical fiber. Optical fibers can be used for measuring a parameter of interest because changes to the environment in which the fiber resides can result in changes to material properties of the optical fiber that are sufficient to alter one or more characteristics of the propagating light such as the amplitude, phase, frequency, spectral content, or polarization of the light. By detecting the change in one or more of these characteristics, a parameter of interest can be measured. Such parameters of interest include the temperature and/or strain of the optical fiber.
Specifically, the use of an optical fiber to measure a parameter of interest such as temperature has many advantages over conventional sensor types. For example, these advantages include high sensitivity, electrical passiveness, immunity from electromagnetic interference, compatibility with high electric or magnetic fields, multiplexing capabilities, and both point and distributed sensing along the length of the optical fiber. Moreover, as optical fibers are also useful for optical filtering, optical fibers used for sensing purposes may be combined with optical fiber based filtering in order to provide both sensing and processing in a common platform, thereby reducing costs and increasing flexibility.
An example of a known method of measuring a parameter of interest utilizing an optical fiber is disclosed in U.S. Pat. No. 4,823,166 issued to Hartog et al. The method described is known as optical time domain reflectometry (OTDR). A parameter of interest is measured by introducing optical energy into an optical fiber and receiving backscattered light returned from various distances along the optical fiber. In OTDR, a pulse of optical energy is introduced to the optical fiber and the backscattered optical energy returning from the fiber is observed as a function of time, which is proportional to a distance along the fiber from which the back scattered light is received. OTDR is also employed in U.S. Pat. No. 5,825,804 issued to Sai.
In U.S. Pat. No. 7,027,699 issued to Tao et al. another type of optical fiber used for sensing temperature utilizes a grating formed in the fiber known as a Bragg grating. When used for sensing temperature, this type of optical fiber is known as an optical fiber Bragg grating sensor (FBG sensor). An FBG sensor comprises an optical fiber with a grating formed transversely in the core by a method such as exposing the fiber to ultraviolet (UV) radiation. The grating produces a differing refractive index within the core of the optical fiber. When light waves propagate along the core, part of the spectrum is reflected by the grating. The reflected wavelength is known as the Bragg wavelength. The Bragg wavelength varies with events and conditions to which the optical fiber is exposed. In particular, the Bragg wavelength will vary with changes in temperature (T) and when the optical fiber is subjected to some form of strain (S). By measuring the Bragg wavelength, the temperature (T) and strain (S) of the optical fiber can be determined. However, it is usually difficult to measure these two parameters independently of each other. In the '699 patent, this problem is overcome by creating a FBG sensor with multiple dissimilar cores in a single optical fiber.
Two basic types of optical fiber grating systems are known in the art including long period gratings and short period Bragg gratings. Short period fiber gratings are generally characterized as having a sub-micrometer period. These types of devices operate by coupling light from the forward propagating core mode to a backward propagating core mode. In general, the short period fiber Bragg grating will have selected narrow band reflection of specific wavelengths. In contrast, long period gratings in optical fibers typically have a period in the range of a few tens of micrometers to 1 millimeter. Such long period gratings promote coupling between forward propagating core modes and co-propagating cladding modes. Long period gratings generally attenuate a certain wavelength and offer wider bandwidths than short period gratings.